1. Field of Invention
The invention relates to an inductive proximity sensor comprising a resonant circuit including two coupled windings.
2. Description of Related Art
Inductive proximity sensors using a resonant circuit are known. See for example U.S. Pat. No. 4,942,372 These sensors function according to the following principle: in the absence of a metallic object near the sensor, the measuring circuit, being constituted by an oscillator inducing resonance, oscillates with a certain amplitude which depends on the own loss of the oscillator (ohmic losses in the circuit""s coil, hysteresis losses in the magnetic circuit of the coil). The proximity of a metallic object causes losses by eddy currents induced in this object and consequently a diminution of the amplitude of oscillation. The comparison of this amplitude with a reference value makes it possible to detect the presence of metallic objects.
The principal disadvantage of this type of sensor resides in the sensitivity to the own losses of the resonant circuit.
When the temperature varies, the own losses of the circuit vary and cause a modification of the amplitude of oscillation.
To assess this temperature dependency, there exist methods using two magnetically coupled coils, such as, the methods described in patents DE 40 32 001 C2 and CH 655 414 A5. However, the implementation of these methods is rather complex.
It is the purpose of this invention to propose a new type of sensor easy to implement and making it possible to remedy the aforementioned disadvantages.